Experimental and theoretical investigation of Al agglomeration process on solid propellant combustion surfaces

Al particles are widely used as metallic additives in solid propellants due to their ability to significantly enhance energetic performance. However, their tendency to agglomerate on the burning surface poses critical challenges, including reduced specific impulse, nozzle erosion, and insulation ablation—factors that limit the broader application of Al-based propellants in engineering practice. In this study, we explore the agglomeration behavior of Al particles at the burning surface of NEPE propellants. By employing high-speed optical imaging, we captured the complete evolution of Al agglomerates during combustion. The agglomerates reached sizes of approximately 500 μm, with secondary agglomeration observed both on the burning surface and after their detachment, contributing to the formation of even larger clusters. A mechanistic explanation for this phenomenon is proposed. While increased ambient pressure did not alter the fundamental agglomeration process, it notably shortened both the formation time and the surface residence time of the Al agglomerates. To describe this behavior, a simplified theoretical model was developed, characterizing the coupled motion of the retreating burning surface and the exposed Al particles. The model predicts a pressure-dependent decrease in agglomeration time, which aligns well with experimental observations. This trend is attributed to the pressure-induced thinning of the condensed layer at the combustion interface. Furthermore, under constant pressure conditions, smaller agglomerates exhibited higher velocities, and elevated pressure further enhanced these velocities. Microscopic analysis of the resulting condensed combustion products (CCPs) confirmed that particle sizes decreased significantly with increasing pressure. Specifically, the median particle diameter declined by a factor of 28.2 as pressure increased from 1 to 5 MPa.